1. Field of the Invention
The present invention relates to a semiconductor device functioning to control a current supplied to a load by a transistor, and a display device including a pixel formed using a current-drive display element of which luminance changes in accordance with a signal, and a signal line driver circuit and a scan line driver circuit which drive the pixel. The present invention also relates to a driving method thereof. Further, the present invention relates to an electronic device having the display device in a display portion.
2. Description of the Related Art
In recent years, a self-luminous display device using a light emitting element such as an electroluminescent (EL) element in a pixel, a so-called light emitting device has attracted attention. As a light emitting element used for such a self-luminous display device, an organic light emitting diode (OLED) and an EL element have attracted attention, and have been used for an EL display or the like. Since these light emitting elements emit light by themselves, they have advantages such as higher pixel visibility, no backlight required, and higher response speed, over a liquid crystal display. Note that the luminance of many of light emitting elements is controlled by the value of current flowing to the light emitting element.
In addition, development of an active matrix display device in which each pixel is provided with a transistor that controls light emission of a light emitting element has been advanced. The active matrix display device is expected to be put into practical use because not only can it achieve high-definition and large-screen display that is difficult for a passive matrix display device, but also it operates with less power consumption than a passive matrix display device.
A structure of a pixel of a conventional active matrix display device is shown in FIG. 46 (Reference 1: Japanese Published Patent Application No. H8-234683). The pixel shown in FIG. 46 includes thin film transistors (TFTs) 11 and 12, a capacitor 13, and a light emitting element 14, and is connected to a signal line 15 and a scan line 16. Note that either a source electrode or a drain electrode of the TFT 12 and one electrode of the capacitor 13 are supplied with a power supply potential Vdd, and an opposite electrode of the light emitting element 14 is supplied with a ground potential.
At this time, when using amorphous silicon for a semiconductor layer of the TFT 12 which controls a current value supplied to the light emitting element, that is, a drive TFT, a change in threshold voltage (Vth) is caused by deterioration or the like. In this case, although the same potential is applied to different pixels through the signal line 15, current flowing to the light emitting element 14 differs from pixel to pixel and display luminance becomes nonuniform among pixels. Note that also in the case of using polysilicon for a semiconductor layer of a drive TFT, characteristics of the transistor are deteriorated or varied likewise.
An operating method using a pixel of FIG. 47 is proposed in Reference 2 to improve the above problem (Reference 2: Japanese Published Patent Application No. 2004-295131). The pixel shown in FIG. 47 includes a transistor 21, a drive transistor 22 which controls a current value supplied to a light emitting element 24, a capacitor 23, and the light emitting element 24, and the pixel is connected to a signal line 25 and a scan line 26. Note that the drive transistor 22 is an NMOS transistor; either a source electrode or a drain electrode of the drive transistor 22 is supplied with a ground potential; and an opposite electrode of the light emitting element 24 is supplied with Vca.
A timing chart showing the operation of this pixel is shown in FIG. 48. In FIG. 48, one frame period is divided into an initialization period 31, a threshold (Vth) write period 32, a data write period 33, and a light emitting period 34. Note that the one frame period corresponds to a period for displaying an image for one screen, and the initialization period, the threshold (Vth) write period, and the data write period are collectively referred to as an address period.
First, in the threshold write period 32, a threshold voltage of the drive transistor 22 is written into the capacitor. After that, in the data write period 33, a data voltage (Vdata) showing a luminance of the pixel is written into the capacitor, and thus Vdata+Vth is accumulated in the capacitor. Then, in the light emitting period 34, the drive transistor 22 is turned on, so that the light emitting element 24 emits light at a luminance specified by the data voltage by changing Vca. Such operation reduces a variation in luminance due to fluctuation in threshold voltage of a drive transistor.
Reference 3 also discloses that a voltage corresponding to the sum of a data potential and a threshold voltage of a drive TFT is a gate-source voltage and current flowing to the TFT does not change even when the threshold voltage of the TFT is changed (Reference 3: Japanese Published Patent Application No. 2004-280059).
In either of the operating methods described in References 2 and 3, the initialization, the writing of a threshold voltage, and the light emission described above are performed by changing a potential Vca several times in each one frame period. In these pixels, one electrode of a light emitting element to which Vca is supplied, that is, an opposite electrode is formed entirely over a pixel region. Therefore, the light emitting element cannot emit light if there is even a single pixel which performs data writing operation besides initialization and writing of a threshold voltage. Thus, a ratio of a light emitting period to one frame period (i.e. a duty ratio) is lowered as shown in FIG. 49.
A low duty ratio requires a high current value supplied to the light emitting element or a drive transistor, which results in increases of a voltage applied to the light emitting element and power consumption. In addition, the light emitting element or the drive transistor becomes easily deteriorated; therefore, much more power is required to obtain a luminance equivalent to that before deterioration.
Further, since the opposite electrode is connected to all pixels, the light emitting element functions as an element with large capacitance. Therefore, more power needs to be consumed to change the potential of the opposite electrode.